The present invention relates to an electrophotographic printer utilizing a means for forming an electrostatic latent image on an image-bearing member, which corresponds to information to be printed, a means for developing the electrostatic latent image with a non-magnetic, one-component developer or a two-component developer containing a magnetic carrier and non-magnetic or slightly magnetic toner, a means for transferring the developed toner image to a recording medium, and a means for heat-fixing the developed toner image to the recording medium, and more particularly to an electrophotographic printer which has reduced thinness and improved portability.
In a conventional electrophotographic printer, an electrostatic latent image corresponding to a printer output (information to be printed) is generally formed on a cylindrical photosensitive drum, brought into slide contact with a magnetic brush of a magnetic developer conveyed on a developing roll containing a permanent magnet roll and disposed opposite the photosensitive drum, so that it is developed (visualized as a toner image). The developed toner image is then transferred onto a recording paper and heat-fixed.
The magnetic developer usable for a magnetic brush method as mentioned above is in many cases a two-component developer consisting of a magnetic carrier and a non-magnetic toner. However, in the case of using such a two-component developer, a concentration sensor and other members are required, failing to satisfactorily miniaturize an electrophotographic printer. Accordingly, a magnetic one-component developer, or a two-component developer consisting of a magnetic toner and a magnetic carrier is mostly used for the electrophotographic printer.
The above-mentioned developers are mostly used in a method where toners are triboelectrically charged by friction between toner particles themselves or between toner particles and a sleeve, etc. However, the development method utilizing the triboelectric charging of the toner involves the problems that the toner tends to agglomerate, and that the image quality varies with environmental conditions. Further, since the magnetic toner contains 20-70 weight % of magnetic powder such as magnetite, etc., it is inferior to the two-component developer containing non-magnetic toner in fixability.
To solve these problems, it has been proposed a developing apparatus permitting the use of one-component developer consisting only of non-magnetic toner. This kind of apparatuses are composed of a toner container which holds a toner, a cylindrical developing roll made of non-magnetic metals and rotatably disposed in the lower part of the toner container, and a blade which is made of flexible materials and has an end fixed to the toner container and the other free end being in slide contact with the developing roll. Incidentally, the developing roll is connected to a bias voltage source and disposed to oppose the photosensitive drum with some gap.
Due to the above-described construction, the toner is charged at a particular polarity by the blade, attracted onto the surface of the developing roll in a form of thin layer due to the electrostatic force, and brought into contact with the photosensitive drum as the developing roll is rotated, thereby developing an electrostatic latent image formed on the photosensitive drum.
Since the developing apparatuses employing a non-magnetic, one-component developer or a magnetic, two-component developer containing a magnetic carrier and a non-magnetic or slightly magnetic toner have a relatively simple structure, electrophotographic printers utilizing these apparatuses can be miniaturized to some extent, but there is a limit in the conventional electrophotographic printer. That is, since the conventional electrophotographic printer has a heatfixing means comprising a heat roll containing a heat source and a pressure roll for pressing the recording paper to the heat roll, and since these rolls are provided with paper-separating fingers and other accessories, it is impossible to reduce the height of the roll pair of the heat-fixing means drastically. Accordingly, it has been impossible to make the conventional electrophotographic printer have an extremely reduced thickness.
However, demands for a portable (hand-carrying) electrophotographic printer as well as for a stationary one have greatly increased recently. Although such a portable electrophotographic printer has already been commercialized for a type of using heat-sensitive papers, the information or image recorded on the heat-sensitive papers will disappear as the time goes. Accordingly, for the purpose of printing information which should be kept for a long period of time, the heat-sensitive paper type electrophotographic printer is not suitable.